Holographic techniques for storing images are well known. Such techniques are commonly used to store images of three-dimensional objects in a variety of different applications. Additionally, various methodologies for utilizing such holographic techniques to store digital data for use in computer systems are currently being explored.
The technique for forming a hologram comprises splitting the highly coherent output beam of a laser into separate reference and object beams. The reference laser beam is directed onto the holographic storage medium, e.g., a photorefractive material, while the object beam is simultaneously directed onto the object whose image is to be stored. Light reflected from the object is directed onto the holographic storage medium, wherein an interference pattern is formed via interference of the reference laser beam with the reflected light of the object beam.
In the case of digital data storage, the object beam passes through an optical modulator, e.g., a liquid crystal spatial light modulator, rather than being reflected off the object to be imaged.
A reconstruction of the originally illuminated object or digital data may be obtained by subsequently directing a reference laser beam onto the holographic storage medium.
A technique generally referred to as angle-multiplexing may be used to store a plurality of such images within a common volume of a holographic storage medium such as lithium niobate. Such angle-multiplexing is discussed in "THEORY OF OPTICAL INFORMATION STORAGE IN SOLIDS," Applied Optics, Vol. 2, No. 4, p. 393 (1963). The method of angle multiplexing generally involves maintaining a constant angle for the object beam, while varying the angle of the reference laser beam for each exposure. Angle-multiplexing thus allows a large number of holograms to be stored within a common volume of holographic storage medium, thereby greatly enhancing the storage density thereof. In the case of digital data storage, such holograms provide high-density storage means suitable for use in computer systems.
The storage of high-efficiency, angle-multiplexed holograms, may, however, result in undesired cross-talk between the stored holograms, as discussed in detail below. As such, it is desirable to provide means by which cross-talk between such holograms may be substantially reduced or completely eliminated.